1. Field of the Invention
This invention relates to a time division switching system for switching first bidirectional lines, each of which conveys a PCM circuit mode channel and a packet mode channel, with PCM circuit mode digital lines and packet mode digital lines.
2. Description of the Prior Art
System of this type are already known in the prior art wherein the first digital lines conveying both PCM coded speech and data packets are digital subscriber lines.
In the system described in French Patent Application No. 2,412,994, a bifilar subscriber line is interconnected between two items of multiplexing and demultiplexing equipment. At the subscriber end, the corresponding equipment is linked to a plurality of packetized data terminals, such as telecopiers and a teleprinter, as well as a telephone set via an analog-to-digital converter. At the other end of the subscriber line, the second multiplexing and demultiplexing equipment is bidirectionally connected, on the one hand, to a packet switching exchange via a link conveying the packet mode channels and, on the other hand, to a conventional telephone exchange, particularly digital, via a link conveying solely speech. Typically, a subscriber line conveys a channel ascribed to speech and two channels ascribed to data packets which are multiplexed, and transmission occurs in two-way alternate mode.
The major drawback of such a system is that the packet switching network must have a number of data terminal equipments (D.T.E.) equal to the number of subscriber lines or equal to the product of this number multiplied by the number of multiplexed packet mode channels in a subscriber line. In view of the fact that the true occupation of such packet mode channels is relatively low for a subscriber, it turns out that the cost of connecting a subscriber line to such a system is high due to the bijective assignment of D.T.E. items with respect to the packet mode channels.
Furthermore, the system according to French Patent Application No. 2,412,944 completely separates the packet switching and the digital voice switching. This implies that the data switching exchange is situated in the same place as the conventional telephone exchange. In fact, considering the forseeably restricted number of subscribers having data terminal equipment, the packet switching network is generally moved away from the subscriber-related telephone exchange, the latter then having to receive the packets along a digital trunk via the switching network so as to cut down on the substructure outlay of connecting subscribers to the packetized transmission media.